can not log NextCloud notes from iOS app when using TailScale

[Weichengzi]

I use NExtCloud behind tailScale. when i want to log in on the NextCloud Notes app, i get the following message:
"Login Failed
The certificate for this server is invalid.
You might be connecting to a server that is pretending to be "xxxxxxxx.ts.net" which could put your confidential inforamtion at risk."

other app like Cookbook, propose a the log in page to avoid this issue.

do you have any advice?

thanks

Steve